Pneumatic systems, such as air brakes and air accessories on heavy trucks, and air powered tools in workshops, can provide reliable power for a variety of purposes. In connection with pneumatic systems, reservoirs or tanks are often used to accumulate and store compressed air so that compressed air is immediately available when required. Because the supply reservoir on vehicles or the storage tank in stationary compressors provides an expansion volume with respect to air that is leaving a compressor, water vapor that is taken in with air at the inlet to the compressor tends to condense in the supply reservoir or storage tank.
It is desirable to prevent moisture from entering pneumatic systems and to remove water that has succeeded in entering such systems for a number of reasons. For example, because liquid water is incompressible, water that collects in a supply reservoir decreases the volume of that reservoir available for compressed air. As a result of the decreased volume, less air is available instantaneously in response to demand, and increased input of compressed air from a compressor may be required. This reduced volume can compromise the safety of brake systems. Where temperatures drop below freezing, valves and other components in the system can stick. Another reason for keeping water out of pneumatic systems is that the reservoirs and other components of a pneumatic system are often made from steel. Accordingly, water in such systems can cause components to rust. The formation of rust can deteriorate components and lead to failure. In addition, rust in the system can contaminate and interfere with the operation of valves, actuators and other components.
In order to prevent water from entering pneumatic systems in the first place, air dryers may be installed. In a typical system incorporating an air dryer, the outlet from the compressor sends compressed air through an air dryer cartridge before that compressed air is provided to supply tanks and the remainder of the system. The air dryer assembly typically contains a cartridge filled with desiccant material. For example, an air dryer cartridge may be filled with a large number of small desiccant beads that the compressed air must pass through before reaching the remainder of the system. In order to prevent the desiccant inside the air dryer cartridge from becoming saturated with water removed from incoming air, the volume defined by the air dryer assembly is periodically purged. For example, purging may be performed when air pressure within the system reaches a predetermined level, for example as determined by a compressor governor, and additional input from the compressor is halted. In general, purging consists of opening an exhaust or purge valve on the air dryer assembly to release collected pressurized air and using that pressurized air to clean the desiccant in the cartridge and to remove moisture from the air dryer assembly. Because the desiccants available for air dryer applications can be used for many cycles of moisture collection and purging, air dryer systems can be effective at preventing moisture from entering pneumatic systems.
However, the ability of an air dryer cartridge to remove moisture from compressed air is adversely impacted by contaminants. In particular, by design, air compressors often introduce oil to the remainder of the pneumatic system with the air compressed by the compressor. Such oil is removed by the air dryer. However, desiccant material that becomes coated with oil is no longer effective at removing moisture from incoming air. Although purge cycles can remove some of the oil from the cartridge, the desiccant material will eventually become loaded with oil, and incapable of preventing moisture from entering the supply tank and the remainder of the system. If the air dryer desiccant material becomes completely coated with oil, oil will begin to be passed into the remainder of the pneumatic system.
The introduction of oil into a pneumatic system is problematic, because pneumatic systems typically incorporate a number of rubber components. These components are degraded by contact with petroleum products, such as the oil typically used to lubricate air compressors. Therefore, failure to timely replace an air dryer cartridge can lead to premature failure of other components in the system. For example, diaphragms, pistons and o-rings can deteriorate, causing leaks in the system and the failure of operating components. As the cost of components incorporated into pneumatic systems has increased, for example as antilock braking systems, vehicle dynamic control systems and the relatively expensive hardware associated with such systems have become more common and because of the safety features that these new systems provide, the need to prevent contaminants in pneumatic systems has increased.
When an air dryer is no longer effective at removing moisture, for example because the desiccant has become approximately 85 to 90% coated with oil, water will begin to collect in the supply tank. The detection of water in the supply tank is an indication that the air dryer cartridge should be replaced. Therefore, operators of pneumatic equipment are usually instructed to drain the supply tank daily (for example nightly), to check for the appearance of water in the stream of air leaving the supply reservoir as it is drained, as an indication that the air dryer needs to be serviced. In actual practice, regular draining of supply reservoirs is often not performed. In particular, the drain valves for supply reservoirs are, by necessity, positioned at or adjacent the lowest point of the supply reservoir. As a result, it can be inconvenient for an operator to access the valve. For instance, in connection with a heavy vehicle, the drain valve may be located underneath the vehicle, or between the vehicle's frame rails. Accessing the drain valve of the vehicle supply reservoir is particularly problematic in connection with vehicles having faired-in chassis, for example in connection with trucks having aerodynamic bodywork, buses or in connection with vehicles having specialized compartments, such as fire trucks. Similar problems can also occur in connection with stationary pneumatic systems having large supply reservoirs and drain valves located close to the floor or other support surface underneath the supply reservoir.
In order to facilitate the draining of supply reservoirs, drain valves allowing remote activation are available. For example, spring loaded valves that can be opened by pulling a lanyard are available. Automatic drain valves that drain the tank periodically, such as by allowing small amounts of air and collected water to exit the tank when pressure in the tank has reached a predetermined amount are also available. However, such systems do not assist in achieving the diagnostic function of observing the discharge from a supply reservoir. In particular, in connection with valves allowing remote actuation, the outlet is typically located some distance from the actuation point. That is, it can remain inconvenient for an operator to observe the discharge from the supply reservoir, even though the action required to open the valve may have been facilitated. Automatic drain valves, because they function autonomously, while the system is in operation, usually operate unobserved. Therefore, the diagnostic function of observing the discharge from a supply reservoir is effectively unavailable when conventional remotely activated drain valves or automatic drain valves are used.
Because operators often do not regularly drain supply reservoirs and/or observe the discharge from such reservoirs, and because of the need to prevent contaminants and oil from entering the pneumatic system, conservative maintenance schedules with respect to air dryer components have been devised. As a result, preventative maintenance schedules can call for frequent replacement of air dryer cartridges in order to prevent damage to other pneumatic system components. However, preventative maintenance schedules based on the passage of time or hours of operation since the air dryer cartridge was last replaced do not necessarily correlate well with the effective life of the air dryer cartridge. For example, needless expense may be incurred as a result of replacing air dryer cartridges more often than is necessary. As another example, such preventative maintenance schedules may not require replacement of the air dryer cartridge soon enough, such as when a compressor is by-passing more than a normal amount of oil.